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Magnetic resonance images were obtained of the lumbar spines of three volunteers in neutral, flexed, and extended postures.To measure migration of the nucleus pulposus within the intervertebral disc during flexion and extension of the spine in living people.Results of experiments on bisected cadaveric spines have indicated that the nucleus migrates posteriorly during flexion and anteriorly during extension in nondegenerate discs. Degenerate discs may have faults or fissures that result in abnormal motion of the nucleus.Proton density weighted, sagittal, magnetic resonance images were obtained from the lumbar spines of three volunteers. Measurements of the positions of the anterior and posterior margins of the nucleus and of flexion and extension angles were made on tracings of the images corresponding to neutral, flexed, and extended postures.The observed frequency (22 of 24 measurements) at which the margins of the nucleus migrated in the directions predicted by results of cadaveric studies was significantly greater than the frequency that would be expected by chance (P < 0.001). The two exceptions may be a result of disc degeneration. There was a significant (P < 0.05) linear correlation between the migration of the anterior margin and the flexion-extension angle and a highly significant (P < 0.001) correlation for the posterior margin and the flexion-extension angle.Flexion of an intervertebral disc in a living person tends to be accompanied by posteriorly directed migration of the nucleus pulposus within the disc. Extension tends to be accompanied by an anteriorly directed migration.